There are three types of antenna configuration presently in use for generating multiple beams that are close together with a high degree of overlap and with side lobes of low level.
A first type of antenna is of the array type with direct radiation, and it uses beam-forming networks that are very complex and that feed a very large number (hundreds or thousands) of radiating sources, each of which is fed by a respective amplifier.
A second known type of antenna uses a parabolic reflector (one for transmission and one for reception) in which each beam is generated by a cluster of 7, 12, or 19 primary sources, the clusters allocated to adjacent beams being caused to overlap by sharing some of the primary sources. The signals that feed the shared individual sources are distributed in transmission and/or grouped together in reception.
The transmission antenna presents a complex beam-forming network suitable for combining a plurality of signals in the primary sources, most of which are shared between adjacent beams.
In receive mode, each element is coupled to a low-noise amplifier and the network is likewise complex.
An antenna of this type using clusters of seven primary sources and operating in the 18.1 gigahertz (GHz)−20.2 GHz band with frequency re-utilization and 108 beams is described in the article by G. Doro et al. entitled “A 20/30 GHz multibeam antenna for European coverage”, published in IEEE—APS Symposium, 1982, pp. 342 to 345.
A third type of antenna avoids this complexity concerning signal generation and the number of primary sources by allocating a single primary source to each beam (so there are thus as many primary sources as there are beams), however that implies no longer using only one parabolic reflector, but instead using three or four parabolic reflectors, each of which generates a plurality of beams. The aperture or diameter D0 of the parabolic reflectors is of the order of 70 λ/HPBW, where λ is the mean wavelength of the band in which the beams are transmitted (or received) by the antenna, and HPBW is the half-power beam width expressed as an aperture angle in degrees, D0 and λ being expressed in the same units. For example D0 may lie in the range 60 centimeters (cm) to 80 cm.
The beams transmitted by the various reflectors are interlaced so as to avoid leaving any gaps between the beams. Such a solution is presently in use for multimedia satellites and it is complex since it requires six to eight antennas (three or four for transmission and three or four for reception).